Lymphocyte Activation Gene-3 (LAG-3) is a negative co-stimulatory receptor that modulates T cell homeostasis, proliferation and activation (Sierro S et al; Expert Opin. Ther. Targets (2010) 15: 91-101). An immunoglobulin superfamily member, LAG-3 is a CD4-like protein which, like CD4, binds to MHC class II molecules, but with two-fold higher affinity and at a distinct site from CD4 (Huard B et al., (1997) Proc Natl Acad Sci USA 94: 5744-9). In addition to exerting very distinct functions (CD4 is a positive co-stimulatory molecule) the two receptors are also differentially regulated. CD4 is constitutively expressed on the surface of all mature CD4+ T cells, with only a small fraction residing intracellularly, whereas a large proportion of LAG-3 molecules are retained in the cell close to the microtubule-organizing centre, and only induced following antigen specific T cell activation (Woo S R et al., (2010) Eur J Immunol 40: 1768-77). The role of LAG-3 as a negative regulator of T cell responses is based on studies with LAG-3 knockout mice and use of blocking anti-LAG-3 antibodies in model in vitro and in vivo systems (Sierro S et al., Expert Opin. Ther. Targets (2010) 15: 91-101; Hannier S et al (1998), J Immunol 161: 4058-65; Macon-Lemaitre L et al (2005), Immunology 115: 170-8; Workman C J et al (2003), Eur J Immunol 33:970-9).
At the cell surface, LAG-3 is expressed as a dimer, which is required for formation of stable MHC class II binding sites (Huard B et al. (1997) Proc Natl Acad Sci USA 94: 5744-9). LAG-3, in soluble form, also occurs in serum of healthy donors and patients with tuberculosis and cancer (Lienhardt C et al. (2002), Eur J Immunol 32: 1605-13; Triebel F et al (2006). Cancer Lett 235: 147-53), and this form may correlate with the number of LAG-3+ T cells (Siawaya J et al. (2008). J of Infection 56: 340-7). The key attribute of LAG-3 as a target antigen for an enhanced lymphocyte depletion agent is its relatively selective expression profile when compared with other agents currently in the clinic, i.e. Campath™ (T/B cells), Amevive (most CD45RO+ T-cells) or Rituxan (B-cells). Few molecules have been identified as sustained markers of in vivo T cell activation in humans. These include LAG-3, OX40, MHC class II, CD69, CD38, ICOS and CD40L. However, apart from LAG-3 and OX40 the majority of these molecules are also constitutively expressed on human natural T regs or on other cell types. LAG-3 is expressed on a small proportion of T-cells in healthy humans (ca. 1-4%), and in a similar proportion of NK cells (Baixeras E et al. (1992), J Exp Med 176: 327-37; Huard B et al (1994), Immunogenetics 39: 213-7). Upon activation with anti-CD3 ca. 30-80% of both CD4+ and CD8+ T cells express LAG-3 within 24 to 48 h; this percentage is increased in presence of IL2, IL7 and IL12 (Sierro S et al, Expert Opin. Ther. Targets (2010) 15: 91-101; Bruniquel D et al (1998), Immunogenetics 48: 116-24). Following antigen-specific stimulation with recall antigen (i.e. CMV or Tetanus toxoid) the majority of activated T cells are LAG-3+. In addition, in humans, LAG-3 is expressed on a sub-population (1-10%) of CD4+ CD25+ FoxP3+ T regs in healthy human blood. This population appears to be functionally suppressive in vitro by cell contact and IL10 dependent mechanisms and therefore may represent a population of recently activated natural or induced T regs [Camisaschi C, Casati C, Rini F et al. (2010). LAG-3 expression defines a subset of CD4+CD25highFox3P+regulatory T cells that are expanded at tumour sites. J. Immunol 184: 6545-51). LAG-3 has been detected on other cell types of hematopoietic lineage, such as plasmacytoid dendritic cells, B-cells, and NKT-cells, but only in the mouse, and mostly following activation (Sierro S, Romero P & Speiser D; Expert Opin. Ther. Targets (2010) 15: 91-101).
Depletion of LAG-3+ T cells may be used to treat or prevent T cell driven immuno-inflammatory disorders. In auto-immune diseases where the majority of auto-reactive cells are chronically activated by self antigens at the disease site and/or re-circulate in the periphery, a short course of a depleting antigen binding protein may selectively deplete this auto-immune T cell repertoire providing long term remission. The precedence for this approach has been demonstrated with the pan-lymphocyte depleting antibody Campath™, in which a single 12 mg injection reduced the rate of relapse by 74% compared to standard treatment in a multiple sclerosis trial (The CAMMS223 Trial Investigators (2008), N Engl J Med. 359:1786-801). Due to the more selective expression of LAG-3 compared with CD52, the target for Campath™, the impact on the naïve and resting memory T cell and natural T regs repertoire should be reduced. This is expected to lead to an improved therapeutic index, maintaining efficacy, but with reduced risk of infection and malignancy as well as onset of auto-immunity associated with Campath™. Additionally, in a baboon tuberculin skin challenge model, the LAG-3 targeting chimeric antibody IMP731 mediated depletion of LAG-3+ T-cells, both in the periphery and at the skin challenge site, resulting in a reduction in the tuberculin skin challenge response (Poirier N et al. (2011), Clin Exp Immunol 164: 265-74). In a further study, a LAG-3 polyclonal antibody depleted LAG-3+ infiltrating T-cells from a rat cardiac allograft and prolonged the survival of these grafts (Haudebourg T et al. (2007), Transplantation 84: 1500-1506).
There exists a need in the art for antigen binding proteins, particularly humanised antibodies, that bind LAG-3 and cause deletion of LAG-3+ activated T cells, and which may have use in the treatment of auto-immune diseases, such as psoriasis, Crohn's disease, rheumatoid arthritis, primary biliary cirrhosis, systemic lupus erythematosus (SLE), Sjögren's syndrome, multiple sclerosis, ulcerative colitis and autoimmune hepatitis; infectious diseases, allergic diseases and cancer.